Efficiency is an important parameter in power applications. In portable electronic device, for example, portable computers, efficiency allows for commercially desirable features such as a smaller battery pack and/or longer battery life. To improve the efficiency of the power supplies in electronic devices designers of power semiconductor devices have endeavored to increase the current carrying capability of the power semiconductor devices without increasing the On resistance value of the same in order to reduce power dissipation, while allowing for the power device to meet high power demands.
Improving the current carrying capability of the device is also advantageous as it allows for the efficient use of the semiconductor material, thereby reducing the cost of the same.
In addition to the current carrying capability certain structural features can improve material use. For example, in power semiconductor switching devices vertical gate design is preferred as it reduces the cell size and thus reduces material consumption. That is, it allows for a greater number of cells per unit area of material thereby reducing the cost of the power semiconductor device without sacrificing performance.
A very well known vertical gate design is a trench-type power semiconductor switching device (e.g. power MOSFET) in which the gate resides within a trench adjacent a base region. In such devices, typically one power electrical contact is formed over one major surface and another power electrical contact is formed over another opposing major surface. Thus, the current path in such devices is through the body of the device. While such devices handle power well, extracting heat from the same is often a design challenge. In addition, the position of the power contacts require packaging considerations that complicate manufacturing. For example, when a power semiconductor device includes two opposing contacts at least two steps are often required for connecting the power contacts to electrical leads of the package. It is, therefore, desirable to have all contacts on one surface. Such a design allows for electrical connection to relevant electrical contacts on one side (which during packaging can be carried out in a single step), while allowing for heat to be extracted from the other side by a heat spreader, heatsink or the like.
Semiconductor power devices which include power contacts on one side of the device and a vertically oriented gate structure are known. Such devices include the advantages described above. A drawback in such devices is that the current path must pass from one contact into the body of the device, under the gate structure and then up toward the second power contact. The long current path contributes to the On resistance of the device.
It is desirable to have a power semiconductor device which does not exhibit the disadvantages of the prior art.